It is common practice in the plastic industry to produce arylene sulfide polymers from sodium hydroxide, sodium hydrosulfide, and p-dichlorobenzene, which are contacted in the presence of a liquid diluent. In U.S. Pat. No. 4,415,729, issued to Scoggins et al. on Nov. 15, 1983, the disclosure of which is incorporated herein by reference, there is disclosed a method of producing polymers from polyhalo-substituted aromatics, alkali metal sulfides and polar organic compounds. Further disclosed in that patent are suitable polar organic solvents which act as diluent materials for the polymerization reaction, and still further, there is disclosed a method for recovering solidified granular poly(arylene sulfide) from a polymerization mixture at a temperature below that at which poly(arylene sulfide) is in a molten state.
After the polymerization reaction is complete, and the polymer has been removed, the liquid polymerization diluent is recovered. In the past, liquid diluent, which preferably comprises N-methy-2-pyrrolidone, (NMP), was recovered in an extraction column, wherein the NMP diluent was separated from a brine mixture comprising mainly water, sodium chloride and NMP. In the NMP extraction recovery process, brine mixture is charged to the upper portion of the extraction column which is equipped with a suitable packing material or conventional perforated trays. An extractant, preferably n-hexanol, is introduced in the lower portion of the extraction column, so as to flow upwardly to extract NMP from the brine feed mixture. Distilled or deionized water is injected into the extraction column at a point above the brine feed to wash the extract, so as to remove traces of sodium chloride. The bottom rafinate, comprising water and dissolved sodium chloride is passed, for example, to a brine well for disposal. The overhead extractant comprising n-hexanol, NMP, a minor quantity of sodium chloride and water carried over with the NMP from the NMP extraction column is passed to fractional distillation for recovery of both organic phases which are then recycled.
Optionally, the brine can be concentrated by partial evaporation of water before it is fed into the extraction column. This optional step enhances the extraction efficiency based on the well known "salting out" effect.
While the above described extraction plus fractional distillation process has been utilized in commercial operation, a problem often arises in that salt carried over with the NMP diluent from the extraction column can cause corrosion in downstream conduits and vessels.
Generally speaking, there are inherent economic considerations for achieving high salt removal from a brine stream an extraction plus distillation operation such that the distillation step is considered to be economical only for processing brine streams having high salt levels and where the distillation unit does not have to produce a high purity stream. Under these circumstances, the distillation can efficiently operate at a lower product purity level.
On the other hand, ion exchange units for absorbing inorganic constituents from aqueous material are considered to be more suitable for effecting separations which produce a high purity product. Accordingly, a salt removal method which effectively integrates extraction plus ion exchange operations would be highly desirable for recovering liquid diluent in a polymerization process.
It is therefore an object of this invention to provide an improved method for recovering polymerization diluent having a low salt content, which employs both extraction and ion exchange operations.
It is a further object of this invention to provide a method of removal of salt from a brine stream which is effective, efficient, safe and economical.
It is yet another object of this invention to recover a liquid diluent stream having a low salt content and which is suitable for recycle in a polymerization process.